I thought that the SD controller board set was also bit slice (the series II internal was of course 8271 based). They handled the same comands at a different I/O location. The only format difference between SD and DD was the DD was proprietary of course and had 52 sectors instead of 26. That made the OS software easy to handle either density rather than creating blocking schemes that CP/M did which would require more software handling. Intel kept ISIS low in memory and had a program load address that did not let you make the OS any bigger than when ISIS first came out. The advantage was that as long as you had 32K of memmory you could run any Intel software and putting 30K more of memory in the system was immediately available (unlike cp/m). The floppy interface for the bit slice was pretty well abstracted in that the cpu wrote commands to I/O ports and the data off disk was dma'd to and from memory. You told it what to do, where to put it, and how much to data. The cpu didn't have to do anything else except wait for completion of the operation. -----Original Message----- From: "Joe R." <rigdonj(a)cfl.rr.com> Sent: Oct 30, 2004 10:20 AM To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk(a)classiccmp.org> Subject: Re: new find: an Intel MDS 800 That's interesting. I've never even heard of a white MDS-800 before. Was it painted white originally or was it painted over an orginal blue one? I think you need a lot more than rewriting the BIOS to handle DD disks. Intels DD controller has a 3000 series bit-slice CPU and some other odd circuitry to handle DD. Joe